An alarm clock of this type is known for example from DE-OS 15 48 133. In order to make possible the display of time over the customary 12 hour face, but to prevent the repetition of the alarm signal after 12 hours, the timing of the alarm is set not by means of a trigger hand on the time display scale, but by a separate dial with a 24 hour division. This, however, is not only highly involved mechanically in view of the design possibilities, but is also unusual for the consumer. Also, because of the technically restricted diameter of such a setting dial, the precise setting of the time of alarm is not possible. The same accuracy problems also arise if the incidence wheel for the signal time coincidence inquiry is laid out for a 24 hour division, i.e., for one revolution per two hour hand revolutions.
In order to trigger the alarm only once in 24 hours and still be able to operate with a setting hand over the 12 hour division of conventional face layouts, a contact wheel is provided in DE 15 23 873, which revolves once in 24 hours and limits the alarm release time by means of the contact setting angle of rotation. However, such a contact wheel must be coupled with the dial train moving in a time keeping mode at a correct angle relative to the contact scanning element at the signal release time. This requires a considerable design effort and a large amount of space for installation.